[IMAPx] Miscellaneous changes
[IMAPx] Miscellaneous changes

- cache_stream in get_message can be NULL, thus count with it and
  do not cause runtime warning from g_object_unref()
- if the whole folder content is removed on the server the local
  folder cache wasn't cleaned up, still showed cached message list

matzipan/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,tintou/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server